Efficacy of Different Pesticides against Fall Armyworm (Spodoptera frugiperda (J.E. Smith) Lepidoptera: Noctuidae) under Laboratory Conditions in Rupandehi, Nepal

Abstract

The fall armyworm (FAW), Spodoptera frugiperda, is a highly destructive pest recently reported in various Asian countries and originated in the subtropical regions of America. It was first recorded in Nepal on May 9, 2019. This invasive species poses a significant threat to maize production because it can undergo multiple generations, migrate, and feed on a wide range of host plants. A laboratory study was conducted with 3 categories of pesticides: conventional, newer, and bio-rational. The study focused on evaluating the efficacy of these pesticides on the third-instar larvae of S. frugiperda. The treatments included (1) newer chemical pesticides: spinosad 45% suspension concentrate (SC) (Tracer 1 ml/3 L of water), spinetoram 11.7% SC (Delegate 0.5 ml/L of water), chlorantraniliprole 18.5% SC (Allcora 1 ml/2.5 L of water), emamectin benzoate 5.7% water dispersible granules (WDG) (top killer 1 g/2.5 L), (2) conventional pesticide: chlorpyrifos 50% EC (Predator 2 ml/L), and (3) bio-rational pesticides: Bacillus thuringiensis var kurstaki 0.5% wettable powder (WP) (Maharashtra 2gm/L), Metarhizium anisopliae 1.0% WP (Kalichakra 5 ml/L), Beauveria bassiana 2% AS (DamanL 5 ml/L), Azadirachta indica 4.5% (Neemix 5 ml/L), Syzygium aromaticum 100% v/v (Kanti herbal Clove oil 3 ml/L), and spinosad 45% SC (Tracer 0.3 ml/L). Fresh maize leaves were treated using the leaf dip method and then fed to 3rd instar larvae of FAW. The results revealed that spinetoram and spinosad caused 100% larval mortality within the first 24 hours after treatment. Similarly, spinosad (99.99%) and clove oil (76.64%) were the most effective bio-rational pesticides followed by B. bassiana and B. thuringiensis. The newer and bio-rational pesticides that showed high efficacy could be suggested for further study in farmers’ fields. They could be recommended for testing as a component of integrated pest management for effective management of FAW in Nepal.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is native to tropical and subtropical regions of South and North America [1]. It invaded Africa in 2016 [2] and entered Asia through Karnataka, India, in May 2018 [3]. It was reported in Nepal for the first time at Nawalpur district in May 2019 [4]. Later, it was confirmed on 12th August 2019 by the National Plant Protection Organization (NPPO) [5]. Soon, it was able to take over 52 districts in Nepal [6].

FAW is a polyphagous pest with a host range of 353 species from 76 families [7]. Based on host plant preference, FAW has been classified under two strains (1) C-strain found in maize and sorghum and (2) R-strain in rice, pasture, and turf grass [8]. Its rate of development depends on diet and temperature [9]. Early stage (1-3^rd instar) larvae feed upon the leaf epidermis, while the later stages (4-6th stage) enter the whorl of the plant and start feeding from the inside [10]. The later stage of larvae is much more destructive as it feeds on the cob, silk, and whorl causing direct plant damage [10].

CABI has estimated that FAW has the potential to cause a loss of 8.3 to 20.6 million tons of maize yield annually in the absence of a proper control method [1]. A recent study estimates a 20–50% maize yield loss due to FAW infestation [11, 12]. Another study showed a 49% average maize yield loss [13]. FAW has the potential to cause 20–25% loss under farmers’ field conditions [14].

Farmers use chemical pesticides irrationally against FAW without considering its feeding habits, damaging nature, and the efficacy of chemical pesticides. In Nepal, conventional synthetic pesticides, which are the most popular ones, are applied at a high dose in the farmers’ fields [15]. At the same time, novel insecticides like spinosad, spinetoram, emamectin benzoate, and chlorantraniliprole are applied at a lower dose [16]. Abusive use of conventional insecticides results in pest resistance and undesirable effects on nontarget organisms [17, 18].

Chlorpyrifos, which met the criteria to be listed as Persistent Organic Pollutants under the Stockholm Convention [19], is used against various foliar and soilborne harmful insects and mites [20, 21]. Anthranilic diamide insecticides like chlorantraniliprole have a novel mode of action to control lepidopteran pests by causing rapid muscle dysfunction and paralysis (RyRs) [22]. Chlorantraniliprole is known to be effective against lepidopteran pests [23] including S. frugiperda [24]. Emamectin benzoate, a fermented product of Streptomyces avermitilis, is known to be effective against various lepidopterous insects including the FAW [25]. Spinosad, derived from the soil bacteria, Saccharopolyspora spinosa, has a unique mechanism that involves the disruption of nicotinic acetylcholine receptors. Spinosyns have a selective effect against target pests and a lesser effect on natural enemies and other nontarget organisms [26]. Spinetoram, an improved form of spinosad, is also isolated from the soil bacteria S. spinosa [27]. Spinetoram is considered to be more effective than spinosad, and it controls a wide range of insect pests [28].

Bio-rational pesticides are those products which are extracted from natural things such as plants, animals, microorganisms, and natural minerals [29]. Their application against insect pests leaves minimal undesirable effects on natural enemies and nontarget organisms [15]. Entomopathogenic fungi like Metarhizium anisopliae [30] and Beauveria bassiana reduce FAW infestation and help increase the crop yield [31]. These fungi might be a good option for the sustainable management of FAW [32]. Bacillus thuringiensis, an effective bio-insecticidal bacterium [33], is widely used against lepidopteran pests [34]. Azadirachtin, an extract from the neem plant, has strong antifeedant, insect growth regulatory, and reproductive effects on lepidopteran insects [35]. Syzygium aromaticum, a rich source of phenolic compounds such as eugenol, eugenol acetate, and gallic acid, possesses great potential as an insecticide [36].

Considering the higher potential risks of conventional insecticides, various bio-rational and newer groups of pesticides were compared in the laboratory. This study aims to find an effective and sustainable management measure for FAW under laboratory conditions. We tried to compare the different groups of pesticides as the geographical nuances in predator-pest dynamics, and the insect strain involved may have different responses towards treatments.

2. Materials and Methods

2.1. Egg Collection and Experimental Site

The experiment was conducted in the Entomology laboratory of Paklihawa Campus, Institute of Agriculture and Animal Science, located at Siddharthanagar, Rupandehi (27.4809°N, 83.4469°E). The FAW egg masses were collected from the National Maize Research Program (NMRP), Rampur, Chitwan (27.6546°N, 84.3508°E). We collected eggs and leaves into a plastic container with a perforated lid and carried them to the laboratory for further study.

2.2. Larval Rearing

The hatched larvae were fed with fresh maize leaves. The larvae in their 3^rd instar stage (length 6–6.4 mm and head capsule width 0.6–0.7 mm) were segregated to use in a bioassay. Visual identification was done using a light microscope (Olympus, Lense:100x). Separate Petri dishes were used to avoid cannibalism [37].

2.3. Experimental Setup

A lab temperature and relative humidity of 26 ± 1°C and 65 ± 5%, respectively, were maintained in the lab which is suitable for FAW development [40]. The overall treatment was divided into two major categories: (1) chemical pesticides and (2) bio-rational pesticides. The doses were determined as per the recommendation by the Nepal Agriculture Research Council (NARC), the chemical manufacturer, and research articles. Treatment details have been mentioned in Tables 1 and 2.

Table 1

Bio-rational treatments along with their trade name, a.i, manufacturers and doses.

Trade name/ai	Active ingredients	Manufacturer	Doses (ml/l)	Group	WHO hazard category
Maharashtra 0.5 WP%	Bacillus thuringiensis (Bt)	International Panaacea Ltd	2 g/L	Microbial pesticide	III
Kalichakra 1.0 WP%	Metarhizium anisopliae	International Panaacea Ltd	5 mL/L	Microbial pesticide	U
DamanL 2.0% AS	Beauveria bassiana	International Panaacea Ltd	5 mL/L	Microbial pesticide	U
Neemix 4.5	Azadirachtin	Golden Agro Chemical Pvt. Ltd	5 mL/L	Botanical pesticide	U
Kanti herbal clove oil BP (100%v/v)	Syzygium aromaticum	Sisla Laboratories	5 mL/L	Botanical pesticide	U
Tracer (45% SC)	Spinosad	Dow AgroSciences Ltd.	1 mL/3L	Extracted from soil bacteria	III
Control	Distilled water	—	—	Not applicable

Table 2

Chemical treatments with their trade name, a.i, manufacturers and doses.

Trade name	Active ingredient	Manufacturer	Doses (ml/l)	Group	WHO hazard category
Tracer (45% SC)	Spinosad	Dow AgroSciences Ltd.	1 mL/3 L	Newer	III
Delegate (11.7% SC)	Spinetoram	Dow AgroSciences Ltd.	1 mL/2 L	Newer	U
Allcora (18.5% SC)	Chlorantraniliprole	B Joshi Agrochem Pharma	1 mL/2.5 L	Newer	U
Top killer (5.7% WDG)	Emamectin benzoate	Ningbo Generic chemical co. Ltd	1 g/2.5 L	Newer	II
Predator (chlorpyrifos 50% EC)	Chlorpyrifos	Dow Agroscience Pvt.Ltd	2 ml/L	Conventional	II
Control	Distilled water	—	—	NA

[38, 39].

The research was carried out in a completely randomized design (CRD). Six treatments of chemical pesticides and seven treatments of bio-rational pesticides were replicated four times. Each treatment consisted of 10 larvae that were kept in a separate Petri dish inside a well-netted box (24 cm × 18 cm × 12 cm). Fresh leaves of maize sown in the field were used for treatment. Before providing maize foliage to the larvae, leaves were sterilized with sodium hypochlorite 1%, washed with distilled water, and then air-dried to remove excess moisture.

2.4. Treatment Preparation

The FAW management recommendations from the combined works of the Plant Quarantine and Pesticide Management Center and the Food and Agriculture Organization of the United Nations were followed to prepare the treatments mentioned in Tables 1 and 2 [41]. Beakers (1 L capacity) were used to prepare stock solutions for each treatment. Each treatment was prepared in 12-hour intervals. The leaf dip method was used to treat the leaves. For the leaf dip method, maize leaves were chopped into pieces and dipped into treatment solution for 10–20 seconds and excess water was removed by keeping the moist leaves on the filter paper and then provided to larvae. Mortality of FAW larvae was recorded every 6 hours and 12 hours for chemical pesticides and bio-rational pesticides, respectively. The larvae were considered dead when they failed to respond when probed with a fine camel hair brush.

2.5. Statistical Analysis

Microsoft Excel was used for data entry. To normalize variance, the larval mortality percentage was arcsine transformed [42]. The mean percentage mortality of FAW larvae was subjected to a one-way analysis of variance (ANOVA). Means were separated using the Duncan Multiple Range Test (DMRT) [42] on R-stat (version 4.0.0) [43] with agricolae package [44] at a significance level of 5%.

3. Results

Both chemical and bio-rational pesticides caused significant ($p<0.05$) mortality over control.

3.1. Bio-rational Pesticide

Among bio-rational pesticides, the highest efficacy (f = 62.20, $p<0.05$) was found in spinosad causing 100% mortality at 36 hours after treatment (HAT) (Table 3). S. aromaticum (100% v/v) was the second effective bio-rational pesticide at 48 HAT causing 95% mortality. Similarly, at 48 HAT, A. indica caused 35% mortality followed by B. bassiana (32.5%), B. thuringiensis (22.5%), and M. anisopliae (25%), respectively. The efficacy of S. aromaticum reached 100% at 60 HAT (f = 45.12, $p<0.05$). B. bassiana caused 87.5% mortality followed by B. thuringiensis (M = 67.5%) at 120 HAT. Azadirachtin and M. anisopliaewere found to be the least effective causing 57.5% and 62.5% mortality, respectively, by 120 HAT (f = 18.61, $p<0.05$) (Table 3).

Table 3

Cumulative mortality of 3^rd instar larva of fall armyworm at different hour intervals after bio-rational pesticide application.

Time/biological pesticide	Mean mortality of third-instar larvae (%)
	12 HAT	24 HAT	36 HAT	48 HAT	60 HAT	72 HAT	84 HAT	96 HAT	108 HAT	120 HAT
Bacillus thuringiensis	7.5c ± 2.5	17.5c ± 6.3	22.5b ± 4.8	22.5b ± 4.8	32.5b ± 7.5	40b ± 10.8	50b ± 7.1	57.5b ± 4.8	65b ± 5.0	67.5bc ± 4.8
Metarhizium anisopliae	0.0c ± 0.0	5cd ± 2.9	12.5bc ± 6.3	25b ± 6.5	30b ± 7.1	32.5b ± 7.5	40b ± 10.8	47.5b ± 11.9	55b ± 15	62.5bc ± 13.8
Beauveria bassiana	10c ± 0.0	12.5cd ± 2.5	22.5b ± 4.8	32.5b ± 4.8	37.5b ± 4.8	50b ± 4.1	60b ± 4.1	70b ± 4.1	72.5ab ± 4.8	87.5ab ± 4.8
Neem	10c ± 7.1	17.5c ± 4.8	25b ± 8.7	35b ± 11.9	37.5b ± 11.8	52.5b ± 16.5	52.5b ± 16.5	57.5b ± 19.3	57.5b ± 19.3	57.5b ± 19.3
Clove oil	60b ± 7.1	70b ± 7.1	90a ± 4.1	95a ± 2.9	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0
Spinosad	90a ± 7.1	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0	100a ± 0.0
Control	0.0c ± 0.0	0.0c ± 0.0	0.0c ± 0.0	0.0c ± 0.0	0.0c ± 0.0	0.0c ± 0.0	0.0c ± 0.0	0.0c ± 0.0	0.0c ± 0.0	0.0d ± 0.0
Grand mean	25.5	32.3	39.9	44.8	48.8	52.1	54.5	57.9	59.5	62.7
F value	29.1	41.6	52.2	54.2	88.4	48.1	48.4	24.6	21.2	18.6
Df	21	21	21	21	21	21	21	21	21	21
LSD	16.5${}^{\ast \ast \ast }$	14.7${}^{\ast \ast \ast }$	14.4${}^{\ast \ast \ast }$	14.1${}^{\ast \ast \ast }$	11.7${}^{\ast \ast \ast }$	15.4${}^{\ast \ast \ast }$	15.0${}^{\ast \ast \ast }$	20.5${}^{\ast \ast \ast }$	21.8${}^{\ast \ast \ast }$	23.4${}^{\ast \ast \ast }$

Note. LSD: least significant difference; ${}^{\ast \ast \ast }$highly significant. Two alphabetical notations denote significant differences between the respective means. The columns represented by the same letter are nonsignificant.

3.2. Chemical Pesticide

Among chemical pesticides, spinetoram showed the highest efficacy (f = 29.93, $p<0.001$) with 90.00% mortality at 6 HAT. Spinetoram was followed by spinosad (87.50%), emamectin benzoate (65.00%), chlorpyrifos (17.50%), and chlorantraniliprole (15.00%). A similar trend of larval mortality was seen at 12 HAT and 24 HAT. Spinosad, spinetoram, and emamectin benzoate killed 100% of larvae as soon as 36 HAT. Chlorpyrifos was able to kill 100% of larvae only after 66 HAT. Newer pesticides like spinosad, spinetoram, and emamectin benzoate proved to be more efficient than older pesticides like chlorpyrifos (Table 4).

Table 4

Cumulative mortality of 3^rd instar larva of fall armyworm at different hour intervals after chemical pesticide application.

Time/chemical pesticide

Mean mortality (%) of third-instar larvae

6 HAT

12 HAT

18 HAT

24 HAT

30 HAT

36 HAT

42 HAT

48 HAT

54 HAT

60 HAT

66 HAT

72 HAT

Spinosad

87.5a ± 2.3

92.5a ± 4.8

97.5a ± 2.5

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

Spinetoram

90.0a ± 4.1

95.00a ± 5

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

Chlorantraniliprole

15.0c ± 5

15.0c ± 5

20c ± 5.8

32.5c ± 4.8

42.5c ± 4.8

47.5c ± 8.5

57.5b ± 11.1

60b ± 9.1

62.5c ± 8.5

65b ± 6.5

75b ± 2.9

82.5b ± 2.0

Emamectin benzoate

65.0b ± 5

87.5a ± 2.5

95.0a ± 2.9

97.5a ± 2.4

97.5a ± 2.5

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

100a ± 0

Chlorpyrifos

17.5c ± 4.8

30.0b ± 4.1

42.5b ± 7.5

55.0b ± 9.6

60.0b ± 10

70b ± 13.5

72.5b ± 11.1

75b ± 8.7

80b ± 10.8

95a ± 2.9

100a ± 0

100a ± 0

Control

0.0d ± 0

0.0d ± 0

0.0d ± 0

0.0d ± 0

0.0d ± 00

0.0d ± 0

0.0d ± 0

0.0d ± 0

0.0d ± 0

0.0d ± 0

2.5c ± 2.5

5c ± 5.3

CV

20.3

23.1

17.8

12.5

12.3

12

11.6

9.4

14.5

10.1

5.3

5.9

Grand mean

45.8

53.3

59.2

64.2

66.7

69.6

71.7

72.5

73.8

73.8

77.4

79.1

LSD

12.1${}^{\ast \ast \ast }$

17.1${}^{\ast \ast \ast }$

15.2${}^{\ast \ast \ast }$

11.6${}^{\ast \ast \ast }$

11.7${}^{\ast \ast \ast }$

11.9${}^{\ast \ast \ast }$

11.7${}^{\ast \ast \ast }$

9.5${}^{\ast \ast \ast }$

15.2${}^{\ast \ast \ast }$

11.0${}^{\ast \ast \ast }$

6.0${}^{\ast \ast \ast }$

6.9${}^{\ast \ast \ast }$

Note. LSD: least significant difference; ${}^{\ast \ast \ast }$highly significant; CV: coefficient of variance; two alphabetical notations denote significant differences between the respective means. The columns represented by the same letter are nonsignificant.

4. Discussion

Bio-rational pesticides are naturally occurring substances that kill pests but cause relatively less harm to humans, microbial flora and fauna, animals, and the environment [29]. In our study, bio-rational pesticides have demonstrated good potential against the mortality of FAW larvae. They have a specific property to affect a wide range of pests in an eco-friendly and sustainable way [31]. In our study, S. aromaticum caused 90% mortality of larvae within 36 HAT. It is the most effective treatment among all the bio-rationalpesticides under study. S. aromaticum contains major biologically active compounds, namely, eugenol (60–90%), eugenyl acetate (2–27%), and β-caryophyllene (5–12%), which have excellent larvicidal activity [45]. It can control the growth, feeding, or reproduction of S. frugiperda larvae [46]. Siddhartha et al. [47] observed similar results with clove oil.

Likewise, B. bassiana and M. anisopliae are entomopathogenic fungi that cause insect mortality by entering the hostinsect's body through the cuticle. B. bassiana killed 87.5% of the larvae after 6 days of treatment. B. bassiana was seen as more effective in causing 3^rd instar mortality than Bt and neem in the 3^rd instar by Kachote [48]. Ramirez-Rodriguez and Sánchez-Peña [49] found 80% insect mortality caused by B. bassiana within 10 days of treatment. Romero-Arenas et al. [50] showed 32% larval mortality at 72 HAT by the commercial strain of M. anisopliae which is similar to our results [31]. M. anisopliae is comparatively less effective in comparison to other bio-rational pesticides. It was experienced by Ramos et al. [32] as well.

B. thuringiensis has been proven to be effective against pests of different orders, especially in Lepidoptera [51]. In our study, it caused 67.5% of mortality at 120 HAT. Kachote [48] has observed similar results to our observations. This slight difference in the strains was explained to be due to their distinct origins, one from Asia and another from Africa.

Azadirachtin-based products do not kill insect pests but disable them [52] and lead to insect mortality. We found that neem caused 57% of mortality at 120 HAT. Sisay et al. [13] found 98.3% mortality within 72 HAT of neem extract. Maredia et al. [53] reported more than 70% mortality of FAW larvae with 1% neem seed oil within 192 HAT. The difference might be due to different recording times.

All the tested chemical and bio-rational pesticides were effective in controlling fall armyworms. Midega et al. [54] and Capinera [10] also reported the effectiveness of these chemicals against FAW. Spinosad and spinetoram were effective against FAW in the present study. These insecticides were described to affect the nicotinic acetylcholine receptors and γ-aminobutyric acid (GABA) receptors in insect nervous systems causing abnormal neural transmission [55]. Sisay et al. [13] indicated that spinetoram can kill 100% of the 2^nd instar larvae within 72 HAT. Siddartha et al. [47] reported that spinosad causes 100% FAW larvae mortality within 48 HAT. Emamectin benzoate has been reported effective against lepidopteran pests like the FAW [34]. It binds to multiple sites (including glutamate and GABA) in insect chloride channels causing a halt in feeding and an irreversible paralysis [56]. It caused 100% mortality within the first 24 hours in our study. Kachote [48] produced a similar result concerning emamectin benzoate. Similarly, chlorantraniliprole is also potent against lepidopteran pests. It causes impaired muscle regulation, paralysis, and ultimately death of sensitive species [57]. The findings from Redmond and Potter [58] and Villegas et al. [59] support our results.

Chlorpyrifos works by inhibiting acetylcholine esterase in nervous tissue leading to the accumulation of acetylcholine and cholinergic hyperstimulation [60]. Gheibi et al. [60] observed 100% larvae mortality within 5 days of treatment. FAW showed moderate resistance to organophosphates like chlorpyrifos in their topical bioassay [61] and unstable resistance towards lepidopteron pests [62, 63]. The result revealed that 100% of mortality was caused by chlorpyrifos only after 120 HAT which is supported in the study conducted by Guillebeau and all [64], in which the efficacy of chlorpyrifos was recorded lower as compared to other chemicals like permethrin, cypermethrin, fenvalerate, esfenvalerate, fluvalinate, and cyfluthrin.

FAW control with a heavy dose of conventional pesticides has numerous drawbacks like resistance to the pest, outbreak of secondary pests, negative impact on a nontarget organism, and environment [54]. The rational use of newer and bio-rational pesticides would be the best option for FAW management. An integrated pest management (IPM) approach including rational and judicial use of newer and bio-rational pesticides might be a good management option against FAW which combines all feasible measures [34].

This study made a statistical comparison among newer, older, and bio-rational pesticides. However, it lacked the estimation of lethal dose against FAW. The estimate would have answered further questions on the efficacy of the treatments. In addition, this work was limited to laboratory conditions. We recommend the replication of our treatments under field conditions.

5. Conclusion

This study concludes that the most effective bio-rational pesticides are spinosad and clove oil, followed by B. bassiana. Chemical pesticides such as spinetoram, spinosad, and emamectin benzoate demonstrate effectiveness in terms of the mortality of 3rd instar FAW larvae under laboratory conditions. These pesticides are recommended for suppressing the population of third-instar larvae FAW. Furthermore, additional fieldwork is necessary to complement these laboratory studies for validation in different larval stages at different times and regions to develop a comprehensive strategy against the fall armyworm.

Acknowledgments

We gratefully acknowledge the Institute of Agriculture and Animal Science, Paklihawa Campus, Tribhuwan University, for providing technical support for this study. We are thankful to Roger Day, Global Advisor, Plant Health, CABI, for providing invaluable suggestions while shaping this article.

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